1. Field of the Invention
The present invention relates to an aqueous ink and a recorded image formed with the ink. More specifically, the present invention relates to an aqueous ink containing at least a coloring material dispersible in water, which has reliability such as satisfactory anti-sticking property, and is capable of providing an image with satisfactory quality and fastness.
2. Related Background Art
Conventionally, various proposals and reports have been made for aqueous ink including hand writing ink and ink jet ink to improve anti-sticking property of ink (the ink does not or hardly form sticking matter) and to improve quality and fastness of the recorded matter.
Conventionally, aqueous ink employs water-soluble coloring materials. Therefore, aqueous ink has satisfactory reliability such as anti-sticking property and long-term stability, and provides recorded images of satisfactory quality. However, image fastness, e.g., resistance to water and light fastness, is not satisfactory with such ink in many cases.
Meanwhile, in order to enhance image fastness such as water fastness and light fastness, attempts have been made to use a coloring material such as carbon black and organic pigments dispersed in an aqueous ink in recent years.
These coloring materials are used in a condition dispersed in an aqueous system by chemical or physical actions of a surfactant or resin. Thus when the pigment in a stable dispersion state is applied to a recording medium, the stable dispersion state is disrupted and the pigment coagulates, which provides the recorded matter with good water fastness and light fastness. On the other hand, recording with such ink tends to reduce saturation of the colored portion owing to the coagulation of the coloring material on the recording medium. Furthermore, when an ink containing a water-dispersible coloring material is used for an ink jet head, evaporation of the water or solvent in the ink occurs near the nozzle of the head, which breaks stable dispersion state of the coloring material causing coagulation thereof. Thus, sticking and clogging will occur in the nozzle.
To solve the above-mentioned problems, Japanese Patent Application Laid-Open No. H08-23191 1 proposes an aqueous ink containing a solid solution obtained by dissolving a hardly water-soluble or water insoluble coloring material in a molten liquid of a water-soluble material which is solid at ordinary temperature, or fine colored particles prepared by making the above solid solution into fine particles of a particle diameter small enough to be stably dispersed in water. This publication describes that when the ink is applied to a recording medium, the solid solution penetrates into the recording material with water, but the water-insoluble fine particles of the coloring material coagulate on the surface of the recording medium to provide a recorded matter excellent in water fastness. Furthermore, this publication describes that near the nozzle of the ink jet head, the coloring material is dissolved in a crystal phase of the solid solution, which prevents precipitation of the coloring material alone. Thus, when fresh ink is supplied from the rear of the nozzle by an operation such as suction, the solid components are solubilized, solving sticking.
In consideration of the stability of the fine particles in the ink using finely powdered coloring material as described above, it is necessary to make the coloring material into fine particles having dimension of, for example, approximately 25 nm. However, if the coloring material is finely powdered, the coloring material is hardly adsorbed to the surface of the recording medium (in other words, permeability of the coloring material fine particles is increased). Thus, it is difficult, in some cases, to obtain high print density. Furthermore, although anti-sticking property may be improved by the use of the above-mentioned solid solution, as drying progresses near the nozzle, the solid component of the solid solution becomes a hard mass. As a result, even if a recovery operation by suction is performed, fresh ink is hardly supplied from the rear of the nozzle. In the worst case, the solidified mass is not removed even if the recovery operation by suction is performed.